java 实现 ItemListener
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Implementing ItemListener
提问by Pickle
i've been having problem in the itemStateChanged section. when I compiled the program i get the "cannot find variable" error and I can't seem to find out where I did wrong. Any help is much appreciated. Thank You.
我在 itemStateChanged 部分遇到了问题。当我编译程序时，我收到“找不到变量”错误，而且我似乎无法找出我做错的地方。任何帮助深表感谢。谢谢。
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class Postcode extends JFrame implements ItemListener {
public static void main(String[] arg) {
JFrame fr = new JFrame("Melaka Postcode");
fr.setSize(240, 125);
fr.setVisible(true);
fr.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
public Postcode() {
String[] code = {"75450", "77000", "78000"};
JLabel lb1 = new JLabel("Postcode");
JLabel lb2 = new JLabel("District?");
JPanel p1 = new JPanel();
JPanel p2 = new JPanel();
JComboBox cb = new JComboBox(code);
cb.addItemListener(this);
Font f = new Font("Verdana", Font.BOLD, 14);
lb2.setFont(f);
p1.add(lb1);
p1.add(cb);
p2.add(lb2);
}
public void itemStateChanged(ItemEvent e) {
if (code.getSelectedItem().equals("75450")) {
lb2.setText = "Bukit Beruang";
}
if (code.getSelectedItem().equals("77000")) {
lb2.setText = "Jasin";
}
if (code.getSelectedItem().equals("75450")) {
lb2.setText = "Alor Gajah";
}
}
}
回答by Dan D.
Your code had a lot of problems, so I have to post here the solution entirely. The fixes:
你的代码有很多问题，所以我必须在这里完整地发布解决方案。修复：
Declared the variables as member variables
将变量声明为成员变量
Called getSelectedItem()on the combobox, not on the array of String
getSelectedItem()在 上调用combobox，而不是在 String 数组上调用
Use the JLabel.setText()correctly
JLabel.setText()正确使用
import java.awt.Font;
import java.awt.event.ItemEvent;
import java.awt.event.ItemListener;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
public class Postcode extends JFrame implements ItemListener {
String[] code = { "75450", "77000", "78000" };
JLabel lb1 = new JLabel("Postcode");
JLabel lb2 = new JLabel("District?");
JPanel p1 = new JPanel();
JPanel p2 = new JPanel();
JComboBox cb = new JComboBox(code);
public static void main(String[] arg) {
JFrame fr = new JFrame("Melaka Postcode");
fr.setSize(240, 125);
fr.setVisible(true);
fr.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
public Postcode() {
cb.addItemListener(this);
Font f = new Font("Verdana", Font.BOLD, 14);
lb2.setFont(f);
p1.add(lb1);
p1.add(cb);
p2.add(lb2);
}
public void itemStateChanged(ItemEvent e) {
if (cb.getSelectedItem().equals("75450"))
lb2.setText("Bukit Beruang");
if (cb.getSelectedItem().equals("77000"))
lb2.setText("Jasin");
if (cb.getSelectedItem().equals("75450"))
lb2.setText("Alor Gajah");
}
}
回答by dbf
In itemStateChanged, you are calling .getSelectedItem()on a variable that is not known to the scope of itemStateChanged(), secondly I don't think you want to call this method on a String array (code), I think you use e.getItem()or e.getItemSelectableinstead.
在 itemStateChanged 中，您正在调用.getSelectedItem()范围未知的变量itemStateChanged()，其次我认为您不想在 String 数组 ( code)上调用此方法，我认为您使用e.getItem()或e.getItemSelectable代替。
See the documentation on ItemEvent
请参阅有关ItemEvent的文档
